A fundamental challenge faced by designers of mass spectrometers is the efficient transport of ions from the ion source to the mass analyzer, particularly through atmospheric or low vacuum regions where ion motion is substantially influenced by interaction with background gas molecules. While electrostatic optics are commonly employed in these regions of commercially available mass spectrometer instruments for ion focusing, it is known that the effectiveness of such devices is limited due to the large numbers of collisions experienced by the ions. Consequently, ion transport losses through the low vacuum regions tend to be high, which has a significant adverse impact on the instrument's overall sensitivity.
Various approaches have been proposed in the mass spectrometry art for improving ion transport efficiency in low vacuum regions. One approach is embodied by the ion funnel device described in U.S. Pat. No. 6,107,628 to Smith et al. Roughly described, the ion funnel device consists of a multitude of closely longitudinally spaced ring electrodes having apertures that decrease in size from the entrance of the device to its exit. The electrodes are electrically isolated from each other, and radio-frequency (RF) voltages are applied to the electrodes in a prescribed phase relationship to radially confine the ions to the interior of the device. The relatively large aperture size at the device entrance provides for a large ion acceptance area, and the progressively reduced aperture size creates a “tapered” RF field having a field-free zone that decreases in diameter along the direction of ion travel, thereby focusing ions to a narrow beam which may then be passed through the aperture of a skimmer or other electrostatic lens without incurring a large degree of ion losses. Refinements to and variations on the ion funnel device are described in (for example) U.S. Pat. No. 6,583,408 to Smith et al., U.S. Pat. No. 7,064,321 to Franzen, EP App. No. 1,465,234 to Bruker Daltonics, and Julian et al., “Ion Funnels for the Masses: Experiments and Simulations with a Simplified Ion Funnel”, J. Amer. Soc. Mass Spec., vol. 16, pp. 1708-1712 (2005).
While the ion funnel device has been used successfully in research environments, its implementation in commercial mass spectrometer instruments may be hindered by issues of cost and manufacturability. A typical ion funnel utilizes approximately 100 ring electrodes, each having a unique aperture diameter. This design results in a high part count and elevated manufacturing cost and complexity. Furthermore, the use of a large number of ring electrodes creates a very high capacitive load, which requires a high-power amplifier to drive the circuit.